1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a zoom lens system suitable for a video camera.
2. Description of the Related Art
As methods of correcting the movement of focus, the zoom lens system has heretofore included an optical compensation type and a mechanical compensation type.
The optical compensation type is of a type for unifying the movement of movable lenses, in which the mechanism is simplified, however, such disadvantages are presented that the lenses become larger in the whole size and the focussing is easily shifted. In contrast thereto, in the mechanical compensation type, the movable lenses move differently from one another, whereby a cam mechanism is required and the lenses become complicated in construction, however, the focussing is not shifted during the zooming and the lenses can be rendered compact in size. Because of the above-described reasons, the zoom lens system of the mechanical compensation type is most widely used.
In the zoom lens system of the mechanical compensation type, a group of variable lenses is moved to change the focal distance, whereby the shift of focussing caused by the movement of the variable lenses is corrected by the movement of a group of compensator lenses, so that, even the group of variable lenses is moved, the shift of focussing on a surface of image-formation is not caused as a whole.
Furthermore, in recent years, according to the mechanical compensation type, a group of focus lenses is disposed in the rear of the group of compensator lenses, whereby the group of focus lenses is moved, so that the range of photographing is enlarged and an object disposed at the closest distance can be focussed. Incidentally, the group of focus lenses is driven independently of the group of variable lenses and the group of compensator lenses, or a value of delivery thereof is mechanically and electrically controlled on the basis of information of the focal distance.
As shown in FIG. 9, in the conventional zoom lens system, a moving locus of the group 4 of the compensator lenses is regulated such that a light flux 2 of an object at infinity, which falls into a group 1 of variable lenses, can be made to focus at a surface 3 of image-formation, and a home position of a group 5 of focus lenses is such that the above-described light flux 2 can be made to focus at the surface 3 of image-formation.
Accordingly, the conventional zoom lens system utilizes only a range of S1.about.S2, which is substantially the half length of the focal depth, whereby, the range, within which focussing is made, is narrow when the power source is turned on, and, when an object disposed at a close distance, which is out of the range, is taken, the object is made to focus at a position shifted rearwardly of the rear end S2 of the focal depth. Because of this, the group 5 of focus lenses moves to a position where focussing is made, whereby much time is taken before focussing, thus presenting the disadvantage that the image is blurred during the photographing time.
Even when the image is blurred during the photographing time, if the zoom lens system applicable to the VTR camera for photographing the movie is used, then the image can be edited after taking, thereby presenting no problem. However, when the zoom lens system is applied to the VTR integrated with a camera, in which the image cannot be edited, a special device is required for removing the blurred image.